1. Field of the Preferred Embodiment(s)
This invention generally relates to ovenized oscillators which provide a stable reference source or frequency in computers or other electronic equipment. Specifically, there is a heater control system that is capable of changing the relative influence of multiple temperature sensors in order to keep a constant temperature at the resonator.
2. Description of the Related Art
Various devices are well known for providing a reference frequency or source such devices are called oscillators. The oscillator typically has a quartz crystal or other resonator and also has electronic compensation circuitry to stabilize the output frequency. Ovenized oscillators heat the temperature sensitive portions of the oscillator which is isolated from the ambient to a uniform temperature to obtain a more stable output. Ovenized oscillators contain a heater, a temperature sensor and circuitry to control the heater. In order to achieve the highest thermal gain, (G.sub.t .ident..DELTA.T.sub.ambient /.DELTA.T.sub.xtal) for an ovenized oscillator assembly, it is necessary to precisely determine the optimum location for the thermistor or temperature sensor that provides feedback to the oven control circuit. With a well designed thermal control circuit, the temperature change at the sensor will be essentially zero. This control point will ideally be located on the same thermal contour or gradient line as the crystal blank or resonator, thus reducing the temperature change that is seen by the crystal to minimize the resultant frequency shift.
If the control point is coupled too tightly to the heat source of the oven, the crystal or thermal load will be under-compensated and the temperature of the load will move in the same direction as the ambient. (i.e. the crystal temperature will increase as the ambient increases.) Conversely, if the sensor is coupled too closely to the ambient temperature, the assembly will be over-compensated and the crystal temperature will decrease as the ambient increases. If this control point or thermal pivot is located at the crystal blank itself, the thermal gain will be maximized.
Since it is not physically practical to locate the sensor on the crystal blank itself, the optimal location external to the crystal must be determined. Determining this point exactly can be difficult, even with thermal modeling of the assembly. Tuning the gain by physical movement of the sensor and repeated temperature testing by trial and error is often necessary. Even after this procedure has been completed on a particular design, unit to unit variations in the thermal characteristics of the assemblies will degrade the thermal gain. By using more than one sensor and properly choosing their locations, using the average output will minimize variations, but the gain may still degrade from unit to unit. Attempts have been made to utilized multiple heaters surrounding the crystal. Unfortunately, these have the same positioning problems as single heaters and incur a cost penalty of the additional heaters. The small size requirements of ovenized oscillators also discourage adding additional heaters. The size requirements for electronic components are decreasing and at the same time circuit densities are required to increase, especially for portable electronics.
Despite the advantages of the prior art oscillators, they have not allowed the user to repeatedly and easily control the temperature profile in an ovenized assembly.